Method for in-line monitoring of via/contact holes etch process based on test structures in semiconductor wafer manufacturing

ABSTRACT

A method for in-line monitoring of via/contact etching process based on a test structure is described. The test structure is comprised of via/contact holes of different sizes and densities in a layout such that, for a certain process, the microloading or RIE lag induced non-uniform etch rate produce under-etch in some regions and over-etch in others. A scanning electron microscope is used to distinguish these etching differences in voltage contrast images. Image processing and simple calibration convert these voltage contrast images into a “fingerprint” image characterizing the etching process in terms of thickness over-etched or under-etched. Tolerance of shifting or deformation of this image can be set for validating the process uniformity. This image can also be used as a measure to monitor long-term process parameter shifting, as well as wafer-to-wafer or lot-to-lot variations. Advanced process control (APC) can be performed in-line with the guidance of this image so that potential electrical defects are avoided and process yield ramp accelerated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/302,809, filed Nov. 21, 2002 now U.S. Pat. No. 6,815,345, claimingpriority to U.S. Provisional Application No. 60/332,016, filed Nov. 21,2001. These applications are incorporated by reference herein for allpurposes.

U.S. application Ser. No. 10/302,809 is related to U.S. ProvisionalApplication entitled “A Method For In-Line Monitoring Of Via/ContactEtching Process Uniformity In Semiconductor Wafer Manufacturing,”Application No. 60/329,917, filed Oct. 16, 2001. The prior applicationis hereby incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to in-line monitoring of via/contactetching process in semiconductor device fabrication by using a scanningelectron microscope (SEM), and more particularly to methods and devicesfor determining whether via/contact holes are over or under etched inthe process of fabricating a semiconductor device.

2. Description of the Prior Art

Very large-scale integrated (VLSI) circuits rely on via/contact holes(as well as trenches) for electrically interconnecting devices ofdifferent layers, interconnecting a layer to an underlying substrate, orinterconnecting a layer to another layer. The electrical defectsassociated with the deep (high aspect ratio) sub-micron via/contactholes take up a significant part of the total yield loss as the currenttechnology approaches to the 0.1 μm node. It is, therefore, essential toensure that the etching process for creating such via/contact holes isoptimized and in-line controlled within the process window, so that thepotential via/contact failure due to either process parameters shiftingor wafer-to-wafer dielectric thickness variation can be identified andavoided in the early steps.

Because of the nonuniformity in the etch rate, and the fact that thefilm itself may be of nonuniform thickness across the wafer or fromwafer to wafer, a certain amount of over-etching is done to ensure thatcomplete etching is achieved everywhere on the wafer, and appropriateelectrical contact is obtained. This is often 10–20% over-etching interms of time past the endpoint point. Even more over-etching (as muchas 50%) may be required when anisotropic processing is done overnon-planar topography. However, as the technology shrinks into the 0.10μm mode, the thickness of over-etch margins have dropped drastically.Excessive over-etch of contact holes will cause the thin metal silicidelayer on top of a drain/source region to be diminished due to thelimited selectivity of the etch process. Also important, there is highprobability that the contact penetrates the shallow pn-junction beneaththe drain/source that leads to high leakage current. For via etchprocesses aiming at open dielectric barriers over the lower metal level,it is also necessary to avoid excessive dielectric barrier over-etch;otherwise, copper is exposed and sputtered during the over-etch step,potentially compromising device reliability.

The integrity of via/contacts can be validated by measuring theresistance of long chains connecting thousands of vias/contacts inseries with each other and located in the scribe lines or in test chipson the wafer. These via/contact chains pass over vanous topographies. Acurrent is forced through these long chains, and the measured voltage isa measure of the average contact resistance. These structures are usedto monitor the via/contact as functional processing conditions andstructures, and to measure lot-to-lot variation. A high value ofresistance in these structures could indicate a problem with under-etch,over-etch, andlor etch residue, but may also be causes of poor metaldeposition, voids in contact region, or other problems incurred insubsequent processes. In addition, this test cannot be performed beforecompletion of the conductive wiring chain. This increases themanufacturing cost.

An electron beam inspection system, or in its simplest form, aconventional scanning electron microscope (SEM), has been proven to be apowerful tool for imaging electrical defects such as via/contact short.As the primary electron beam scans over the inspection area, low energysecondary electrons (SE) (˜5 eV) will be generated from the surface andcollected by the SE detector to form an image. Due to the differences inSE yields of the involved materials or the abnormal electricalconductivity of the defect portions, the inspected surface will beunevenly charged positively and/or negatively. Negatively chargedsurfaces tends to produce more SE to the signal detector, thus itsappearance is relatively brighter, while a positively charged surfaceattracts more SE and thus appears relatively darker. This is theso-called voltage contrast (VC). VC can be used roughly to dividevia/contact holes into the categories of under-etch or over-etch.However, it lacks the sensitivity to the level of under-etch orover-etch, thus is not suitable for process monitoring.

SUMMARY OF THE INVENTION

An objective of this invention, therefore, is to provide a method andtest structures to monitor the via/contact etching uniformity over adielectric layer of a wafer.

Another objective of this invention is to provide a method and teststructures to monitor the process variation due to etching parametersshifting and/or dielectric layer thickness variation from wafer-to-waferor lot-to-lot.

A further objective of the present invention is to provide a method andtest structures to estimate the amount of over-etch or under-etch in theactual device region with respect to the just-etch.

In accordance with the above-described objects and those that will bementioned and will become apparent below, a test structure formonitoring the via/contact holes includes the provision of via/contactholes of different sizes and densities formed into a dielectric layerfor making contact to the buried conductive layer or active regions suchas source/drain. The thickness of the dielectric layer as well as itstopography resembles that required in the functional dies for makingactual devices. FIG. 1 depicts one of such layout as an array withdensity varying along columns 101 and hole size varying along rows 102.The test structures may be placed on the semiconductor wafers as“drop-ins,” which are located where functional dies would normally beplaced. Or they can be placed in wafer scribe lines, which are linesbetween functional dies defining diamond saw cuts that separate finisheddevices. The via/contact holes in the test structure, formedsimultaneously with those in the functional die, will be etched todifferent levels as the result of the microloading effect or RIE “lag”.With reference to the related patent application referenced above,voltage contrast of these holes will reverse at certain beam conditionsfrom bright to dark if the thickness of under-etch remains over athreshold value at a certain primary beam energy and current, as shownin FIG. 2. Curve 201 depicts the SE signals (normalized to backgroundsignal) originating from the via/contact hole bottom as a function ofthickness of remains or recesses with respect to just-etch. Point 202corresponds to the threshold thickness across which the contrast of thehole reverses. If the test structure is properly designed in such a waythat the etching variations inside the hole ranges from under-etch toexcessive over-etch, for instance, from −150 Å (over-etch) to 100 Å(under-etch), the VC contrast of these holes will experience atransition from bright to dark. For the test structure in FIG. 1, thecorresponding VC image may have a similar appearance as shown in FIG. 3.Via/contact holes at the lower-right corner are of smaller sizes andrelatively higher densities, thus turn up brighter due to the relativelyslow etching rate associated with microloading effects, while for thoseat the upper-left corner having relatively larger holes and lowerdensity, turn out to be darker as over-etching commonly happens. Thereis a narrow transition region lying between the upper-left andlower-right corners, at which image signals are so sensitive to theactual remains that holes may appear white or dark, depends on therandomly thickness variation.

The foregoing VC transition image characterizes the etching process asits size and location should are generally fixed for a given process.Proper image processing, for instance, by subtracting two similar imagesof the adjacent test structures, may highlight the VC transition portionin the resulting image. This resulting image can be regarded as a“fingerprint” image of a specific etching process. Shifting or change ofthis fingerprint image may happen over different parts of a wafer, orfrom wafer-to-wafer and lot-to-lot. The former implies the occurrence ofnon-uniform etching because of the process itself, or uneven dielectricthickness over the wafer; caution must be paid if the shifting exceedsthe predetermined tolerance. The latter is due to long term processparameter shifting; manual or automatic process adjustment is necessaryto keep it within the tolerance region.

According to the related patent application referenced above, at a givenprimary beam energy and current, the VC transition happens at a certainunder-etch level of around a certain thickness. In fact, the VCtransition region can be treated as a curve of equi-thickness ofunder-etch, usually on the order of several tens of angstroms. Itsconformal curves may be used to characterize the etching similarity interms of the thickness of under-etch or over-etch. Calibration can bemade further to quantize these equi-etching lines with respect to thejust-etch.

FIG. 4 shows an example of the calibrated equi-thickness curves over theprocess “fingerprint” image. For a given via/contact design in thefunctional die region, it is always possible to locate its position inthis quantized fingerprint image and check out its corresponding amountof over-etch thickness. System fine tuning or APC can be made to controlthe percentage of over-etch time so that the amount of over-etch istightly controlled not far away from just-etch, for instance, 50 Å. Bydoing so, the presently required over-etch time when etching contactholes can be reduced considerably by 10%.

IN THE DRAWING

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 is an illustrative layout of the test structure and consists ofarrays of via/contact holes of different sizes and densities;

FIG. 2 illustrates the detected SE signals from a via/contact holebottom normalized with the background signal as a function of thicknessof under-etch and/or over-etch;

FIG. 3 shows the voltage contrast image of the test structure undercertain beam conditions, and demonstrates the transition of via/contactcontrast due to uneven etching ranging from under-etch to over-etch;

FIG. 4 depicts the fingerprint image of the test structure and theoverlapped equi-thickness lines of under-etch and/or over-etch; and

FIGS. 5A and 5B illustrate the shifting of fingerprint images towardsexcessive over-etch and under-etch, due to either etching parametersshifting or wafer-to-wafer variation, and its implications on thevia/contact quality in the functional die region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts an embodiment of a via/contact test structure consistingof arrays 103 with increasing via/contact size along the row 101direction and increasing densities or pattern factor (exposed Siarea/wafer area) along the column 102 direction. Each via/contact arrayconsists of via/contact holes of a single size and density (patternfactor), as 104 indicates, and is etched simultaneously with thevia/contacts in the functional dies into a dielectric layer of similarthickness and topography for making contact to the buried conductivelayer or active regions such as source/drain. The aim of the embodimentis to have the etching process impose different etch rates, as theresult of the microloading effect, and/or RIE lag, over the variantholes and to get uneven etch over the structure range from under-etch toover-etch. Other forms or modifications and/or embodiments may also meetthis purpose and are thus intended to be covered by this disclosure. Theembodiment of such test structure involves numerousimplementation-specific decisions for achieving the ultimate goal, suchas compliance with litho-related constraints. It is necessary to ensurethat the smallest size of via/contact, usually 10% below the design sizein the functional dies, is within the exposure process window, so thatno significant litho-induced size variation is transferred to theetching step. The test structures may be placed on the semiconductorwafers as “drop-ins,” which are located where functional dies wouldnormally be placed. Or they could be placed in wafer scribe lines, whichare lines between functional dies defining diamond saw cuts thatseparate finished devices.

The foregoing description disclosed the test structure and theanticipated etching results. Further embodiments of this invention relyon voltage contrast imaging of this test structure with a SEM, or thelike, apparatus. According to the related patent application referencedabove, under and/or over etch via/contact holes will appear differentlyin contrast in a SEM image due to their voltage differences induced byprimary electron beam irradiation. In general, primary electronirradiation will cause the surface to be positively charged ornegatively charged depending on the total electron yield associated withthe material as well as the immediate field conditions, such asextraction and retarding fields. The sustained positively charges tendto attract the consequent SE back to the surface, thus the correspondingfeatures appears relatively dark while the negative charged surfacesrepulse the SE so that the features appear relatively bright. Thedetected SE signal is a function of the surface charging voltages, orequivalently in the contact case, the remaining SiO₂ thickness.

FIG. 2 illustrates the detected SE signals as a function of thethickness of under-etch (positive thickness) and/or over-etch (negativethickness). The signals are normalized with background level taken fromthe top surface, thus values higher than one are equivalent to a brightimage and those lower than one are equivalent to a bright image. Thesecurves are obtained at a charge equilibrium condition at which theoverall voltage contrast is stable. Curve 201 corresponds to the casewhere the substrate pn-junction is equivalently forward-biased, as thecompound effects of an external field associated with the electronoptical system and local charge induced field associated with the highaspect ratio hole. For instance, for a p-type substrate with n-typesource/drain under a weak enough extraction field, the contact hole actsas a Faraday cup and traps SE within the sidewall and bottom. Thelocally charged induced field predominates and imposes forward bias onthe pn-junction. Negative charges on the contact bottom will not sustainas they always find their way to release through the low resistantpn-junction. As the result, under-etch appears bright and over etchappears dark. However, this voltage contrast image is reversible if theexternal extraction field is strong enough to overwhelm the local chargeinduced field. Once the effectively biased field for the pn-junctionbecomes reversed, the relatively high resistance pn-junction preventsthe negative charges on the contact bottom from releasing, thus thethrough contact appears bright. For under-etch contacts with slightremainder, some electrons among the generated electron-hole pairs in thesubstrate may penetrate the thin SiO₂ barrier to the bottom surface bythe tunneling effect, the detected SE signal decreases exponentiallywith the increasing SiO₂ thickness. As the thickness further increases,the probability for the generated SE to escape the holes increases asthe aspect ratio decreases. The SE signal in the reverse-biased case isdepicted by curve 203.

It mat be noticed from FIG. 2 that there is a threshold thickness beyondwhich via/contact appearance will transit from one contrast to itsopposite. The sharp slope across the threshold point 202 implies thatthe VC image can be very sensitive to the actual thickness ofremainders. Slight variation of under-etching remainders around thethreshold value may result in a significant difference in contrast orsize. Experiment has confirmed this predication and proper tuning of theprimary electron beam energy to 200 eV and current to 75 nA will shiftthe threshold thickness to the order of several tens of angstrom. Thisreduced threshold thickness provides a valuable measure for monitoringthe etching process towards the just-etch or minor over-etches. Thedesign of the test structure aims to produce such kind of etching leveland covering from −150 Å over-etch to 100 Å under-etch.

FIG. 3 illustrates the VC image of the test structure. Via/contact holesat the lower-right corner are etched at a relatively slow etching rateand stop at under-etch with relatively more remainders, while those atthe upper-left corner are of relatively larger hole size and lowerdensity; so that it results in excess etch ending inside the siliconsubstrate. As a result, the former is associated with bright holes whilethe latter is associated with dark ones. In between the upper-left andlower-right corners, a narrow transition region exists in which theholes turn out to be either bright or dark and merely depends on theunavoidable random thickness variation in an etching process. For agiven primary beam energy and current, this transition regioncharacterizes the etching process by its relative size and location inthe VC image of the test structure as it is generally fixed for a givenprocess. Proper image processing, for instance by subtracting twosimilar images of the adjacent test structures, may highlight thetransition portion in the resulting image, as shown in FIG. 4 by theshadow region 401. This resulting image is also called process an IDmap, or process “fingerprint”. Shifts and/or changes of this fingerprintmay happen over different parts of a wafer, or from wafer (lot)-to-wafer(lot). The former implies the occurrence of non-uniform etching becausethe process itself mismatches the specs, or uneven dielectric thicknessover the wafer; caution must be taken if the shifting exceeds thepredetermined tolerance. The latter can be attributed to long-termprocess parameter shifting; manual or automatic process adjustment isnecessary to keep it within the tolerance region.

At a given primary beam energy and current, the VC transition happenswithin a narrow under-etch region with the thicknesses of remaindersvarying around the primary beam determined threshold thickness. In otherwords, the thickness of remainders of a via/contact can be determinedfrom its corresponding transition image at known beam conditions. Forinstance, if the primary electron beam is of 400 eV in energy and 60 nAin current, the transition region 401 represents that an equi-thicknessregion of about 50 A remained within via/contacts. Further calibrationcan turn this roughly equi-thickness region into equi-thickness line402, as shown in FIG. 4, with the x-coordinate representing patternfactor, and the y-coordinate representing the via/contact sizes. Othercurves approximately conformal to this line extend the equi-thicknessline to the nearby over-etch and under-etch regions. By doing so, aprocess “fingerprint” image is quantified with the amount of under/overetch with reference to the just-etch. For a given via/contact designrule in the functional die region, it is always possible to locate itsposition in this quantified “fingerprint” image and find itscorresponding amount of over-etch thickness. With the aid of thequantified “fingerprint” image, manual or Automatic Process Control(APC) can be made simple by controlling the percentage of over-etch timeso that the amount of over-etch is tightly controlled not far away fromjust-etch. For example, for 0.18 μm via/contacts in the functional dieswith a pattern factor of 2%, one can easily locate its correspondingposition 403 in the fingerprint image of FIG. 4. The average thicknessof over-etch is found to be around 50 A, which is within the acceptablerange. If wafer-to-wafer dielectric layer thickness variation or processshifting results in a change of fingerprint image, as shown in FIG. 5A,excessive over-etching up to 90 A is recognized, referring to 501. APCshould be acknowledged, for instance, to reduce the over-etching time bya certain percentage, or to increase the etching selectivity properly.If the fingerprint image shifts in the opposite direction as shown inFIG. 5B, the via/contact holes are most probably under-etched orinsufficiently over-etched, referring to 502. Both cases may incur ahigh contact resistance thus are beyond the process window. APC shouldrespond to this variation with longer over-etching time. The practicalways and parameters for the process adjustment in response to theprocess shifting and wafer-to-wafer variations depend on the devicedetails under processing and vary from system-to-system and fab-to-fab.However, should the process parameters shift beyond the tolerance, orthe dielectric layer thickness variation exceed the process window, thefingerprint image will immediately reveal this abnormality, and provideguidelines for the consequent control. This inline APC considerablyreduces the final electrical failure rate and accelerates the yield rampfor new products. Also, by doing so, the presently required over-etchtime when etching contact holes can be reduced significantly, forinstance by 10% or higher.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalent,. and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

1. A method for characterizing an etching process, the methodcomprising: performing an etching process to a first layer to form anarray of via regions and to a second layer to form at least a first viaoutside the array of via regions, the first layer associated with afirst thickness, the second layer associated with a second thickness;obtaining an image of the array of via regions; determining a pluralityof via regions in the array of via regions, each of the plurality of viaregions includes a second via and a third via corresponding to a darkarea and a bright area on the image respectively; processing informationassociated with the plurality of via regions and the first via;determining a first depth associated with the first via relative to thesecond thickness based on at least information associated with theplurality of via regions and the first via; wherein: the processinginformation associated with the plurality of via regions and the firstvia includes determining a threshold depth with respect to the firstthickness; the threshold depth is related to a transition from a darkregion to a bright region on the image.
 2. The method of claim 1wherein: the first layer and the second layer are associated with asubstantially same composition; the first thickness and the secondthickness are substantially the same.
 3. The method of claim 1 whereinthe image of the array of via regions is a voltage contrast image. 4.The method of claim 1 wherein the obtaining an image of the array of viaregions comprises obtaining the image with a secondary electronmicroscope associated with a primary electron beam.
 5. A method forcharacterizing an etching process, the method comprising: performing anetching process to a first layer to form an array of via regions and toa second layer to form at least a first via outside the array of viaregions, the first layer associated with a first thickness, the secondlayer associated with a second thickness; obtaining an image of thearray of via regions; determining a plurality of via regions in thearray of via regions, each of the plurality of via regions includes asecond via and a third via corresponding to a dark area and a brightarea on the image respectively; processing information associated withthe plurality of via regions and the first via; determining a firstdepth associated with the first via relative to the second thicknessbased on at least information associated with the plurality of viaregions and the first via; wherein the obtaining an image of the arrayof via regions comprises obtaining the image with a secondary electronmicroscope associated with a primary electron beam; wherein theprocessing information associated with the plurality of via regions andthe first via comprises: processing information associated with theprimary electron beam; determining a threshold depth with respect to thefirst thickness based on at least information associated with theprimary electron beam; wherein the threshold depth is related to atransition from a dark region to a bright region on the image.
 6. Themethod of claim 5 wherein the determining a threshold depth includesdetermining the threshold depth based on at least information associatedwith a primary beam energy and a primary beam current.
 7. The method ofclaim 5 wherein the threshold depth is equal to about 50 Å.
 8. Themethod of claim 5 wherein: the processing information associated withthe plurality of via regions and the first via includes determining afirst plurality of pairs of via sizes and via densities; each of thefirst plurality of pairs is related to the threshold depth.
 9. Themethod of claim 8, and further comprising: determining a secondplurality of pairs of via sizes and via densities; wherein: each of thesecond plurality of pairs is related a second depth; one of the secondplurality of pairs includes a first via size and a first via densityassociated with the first via.
 10. The method of claim 7 wherein thefirst depth is equal to the second depth.
 11. The method of claim 1, andfurther comprising adjusting one or more parameters associated with theetching process based on at least information associated with the firstdepth of the first via.
 12. The method of claim 11 wherein the one ormore parameters include an etching time.